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Zorluk: Çok zorOSPFv2 Network Types and DR/BDR Selection

An enterprise network features five routers participating in OSPFv2 on a shared Ethernet segment (192.168.12.0/24). A network engineer issues the command `show ip ospf neighbor` on router R3 and observes the following status output:

text
Neighbor ID Pri State Dead Time Address Interface
10.10.10.1 100 FULL/DR 00:00:35 192.168.12.1 GigabitEthernet0/0
10.10.10.2 50 FULL/BDR 00:00:38 192.168.12.2 GigabitEthernet0/0
10.10.10.4 0 FULL/DROTHER 00:00:31 192.168.12.4 GigabitEthernet0/0
10.10.10.5 200 FULL/DROTHER 00:00:36 192.168.12.5 GigabitEthernet0/0

If router 10.10.10.1 unexpectedly suffers an interface failure on GigabitEthernet0/0, which state transition will immediately occur on the segment?

  1. Router 10.10.10.2 is promoted to Designated Router (DR), and Router 10.10.10.5 is elected as the new Backup Designated Router (BDR).Cevap
  2. B
    Router 10.10.10.5 immediately becomes the new Designated Router (DR) due to its highest interface priority of 200, while Router 10.10.10.2 remains the BDR.
  3. C
    Router 10.10.10.2 becomes the new Designated Router (DR), and Router 10.10.10.4 is elected as the new BDR based on its router ID.
  4. D
    Router 10.10.10.5 becomes the new Designated Router (DR), and Router 10.10.10.4 is promoted to BDR.

Cevap

Router 10.10.10.2 is promoted to Designated Router (DR), and Router 10.10.10.5 is elected as the new Backup Designated Router (BDR).
In OSPF multiaccess networks, DR/BDR election follow two strict rules during failures: (1) When the DR goes down, the active BDR immediately assumes the DR role to maintain stability and prevent network re-convergence delays. (2) An election is subsequently conducted to fill the vacant BDR position. Since router 10.10.10.2 was the BDR, it becomes the DR. Among the remaining routers, router 10.10.10.5 has an interface priority of 200 (higher than remaining eligible routers), while router 10.10.10.4 has a priority of 0 (ineligible). Thus, router 10.10.10.5 is elected as the new BDR.

Adım Adım Çözüm

1
Analyze DR/BDR state transition upon DR failure
Router 10.10.10.2 (the current BDR) is automatically promoted to the Designated Router (DR) position.
OSPF DR/BDR election mechanics specify that whenever the current DR fails, the existing BDR takes over the DR role immediately without triggering a full DR election.
2
Evaluate eligibility for the newly vacant BDR position
Routers 10.10.10.4 (Priority 0) and 10.10.10.5 (Priority 200), alongside R3, are remaining neighbors.
Router 10.10.10.4 is configured with an interface priority of 0 (`ip ospf priority 0`), making it ineligible to become DR or BDR.
3
Determine the winner of the new BDR election
Router 10.10.10.5 wins the election and becomes the new BDR.
Among all eligible candidate routers for the vacant BDR role, Router 10.10.10.5 has the highest interface priority (200).

Anahtar Kavram

OSPFv2 DR/BDR Election Mechanics, Non-preemption, and Priority 0 Ineligibility
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Konular

OSPFv2 Network Types and DR/BDR Selection

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